I'm also in the same boat... shocks are available but around £100 a pop, £400+ quid for a set of front & rear is a bit steep.
At that price I'm thinking of switching to adjustable shocks so I can adjust the ride quality - ideal when I've swapped over to larger wheels and lower profile tyres.
I checked around the usual suspects, Koni, Spax etc. but not surprisingly the Fiat 850T/900T doesn't appear. I've sent enquiries to various companies but not had anything back from them yet.
I have seen photos of a conversion with Spax ages ago so they must exist (still trying to find the site).
Has anyone fitted adjustable dampers or know of a cheaper supplier of standard shocks?